Known in the art is an apparatus for automatically controlling the active power produced by the generator of a hydraulic turbine-generator unit, comprising a reference signal source for developing an assigned power signal, power measuring means responsive to the active power produced by the generator of the hydraulic turbine--generator unit, gate opening sensing means responsive to the degree of opening of the hydraulic turbine gate apparatus, gate opening control means for controlling the degree of opening of the gate apparatus, and an adder having its output connected to the gate opening control means, one of its inputs connected to the gate opening sensing means and its another input connected via a PID signal forming means and a second adder, to the reference signal source and to the power measuring means, the reference signal source and the power measuring means being connected to different inputs of the second adder. The PID signal forming means consists of an integrator, an amplifier and a differentiator connected in parallel (cf. Bulletin No. 07058 "Electric Governor for Hydraulic Turbines" of the Woodward Governor Company, p. 24, FIG. 25). Since the amplifier and the differentiator of the PID signal forming means have small amplification factors, the latter, in the frequency range corresponding to the bandwidth of the power control circuit, has a frequency response similar to that of an integrating element.
The proportional feedback between the gate opening sensing means and the gate opening control means in such a control apparatus makes it possible to avoid instability caused by the presence of two integrating circuits, one of which is the PID signal forming means and the other is the servomechanism of the gate opening control means.
Such a control apparatus, however, has a low response speed, which is due to the fact that the assigned power signal is applied to the gate opening control means via an integrating circuit.
To improve the response speed there was proposed an apparatus for automatically controlling the active power of the generator of a hydraulic turbine-generator unit, comprising a reference signal source for developing an assigned power signal, power measuring means responsive to the active power produced by the generator of the hydraulic turbine-generator unit, gate opening sensing means responsive to the degree of opening of the hydraulic turbine gate apparatus, gate opening control means for controlling the degree of opening of the hydraulic turbine gate apparatus, and frequency-responsive summing means having its inputs respectively connected to the reference signal source, to the power measuring means and to the gate opening sensing means, and its output connected to the gate opening control means so that a variation in a certain sense of the active power produced by the generator or of the degree of opening of the gate apparatus cause a variation of the signal at the output of the frequency-responsive summing means in the direction opposite to the variation of said signal caused by a variation of the assigned power in the same sense, wherein the frequency-responsive summing means comprises an integrator having one of its inputs connected to the reference signal source and another of its outputs connected to the power measuring means, and an adder having its inputs respectively connected to the reference signal source, to the output of the integrator and to the gate opening sensing means, and its output connected to the gate opening control means (cf. the U.S.S.R. Author's Certificate No. 379,013).
The control apparatus has two closed control circuits, wherein the assigned power signal is respectively compared with the signal corresponding to the actual active power produced by the generator and with the signal corresponding to the degree of opening of the gate apparatus. The first control circuit has a slow response, which is due to the presence of the integrator, the latter being necessary for preventing the gates of the gate apparatus from movement with the swings of the generator power output which take place in the course of operation of a generator connected to a power system and for preventing the generator power output from falling or rising when a connection or a disconnection of consumer loads occurs. The second control circuit has a quick response and makes it possible to avoid instability caused by the presence in the control apparatus of two integrating circuits, one of which is the integrator of the first control circuit and the other is the servomechanism controlling the degree of opening of the gate apparatus.
In this control apparatus the signal corresponding to the assigned power is applied to the input of the adder not only through the integrator but also directly. As a result, change in the assigned power signal first brings into operation the quick response control circuit comprising the gate opening sensing means, whereupon the generator power output is corrected by the slow response control circuit comprising the power measuring means. In this manner a more rapid response of the control apparatus is provided without a reduction in static control accuracy.
In this case, however, the response speed of the control apparatus also remains relatively low. This is due to the fact that a change in the assigned power signal leads to the appearance of a voltage at the integrator input, as a result of which, the integrator output voltage immediately begins to grow. The growth of the voltage at the output of the integrator continues until the actual generator power output attains the assigned value. The integrator output signal passes to the input of the adder and is added to the assigned power signal. As a result of this, the gate opening control means moves the gates of the gate apparatus to a position corresponding to the steady-state generator power output exceeding the assigned value. The return of the gate apparatus to the position corresponding to the assigned power value occurs when the signal at the integrator output goes down, i.e., after the actual power produced by the generator has risen above the assigned value. Thus, in the course of operation of the control apparatus an overshooting takes place causing a dynamic error. This error is the greater, the smaller is the time constant of the integrator. Therefore the time constant of the integrator is such a control apparatus is chosen sufficiently great so that the dynamic error might not exceed the admissible value and usually amounts to several tens of seconds. Such a great time constant, however, leads to a lower response speed of the control apparatus.